1. Field of Invention
The invention relates to an operating current modifying device and an operating current modifying method, and, in particular, to an operating current modifying device and an operating current modifying method, which are applicable to an optical disk drive.
2. Related Art
The power generating element of an electric device is easily affected by temperature. For example, the performance and accurate of the electric device may vary due to thermal factors. In order to control the power generating element at different temperatures, a proper control module must be designed to modify the effects of thermal factors.
In order to control the output power of the power generating element at different temperatures, a proper control module is designed in the power generating element. Herein, a power feedback signal, which is generated when the power generating element generates the output power, is applied to control the operating currents for the power generating element to generate the output power at different powers.
Referring to FIG. 1, when the temperature of the power generating element is at a temperature T1, the output power and the operating current of the power generating element are related in a line L1. In other words, when the operating temperature of the power generating element is at the temperature T1 and the operating current is at a current I1, the power generating element generates the output power at a power P1. If the operating current is at a current I2, the power generating element generates an output power at a power P2.
If the temperature of the power generating element rises to a temperature T2, the operating current equal to the current I1 will drive the power generating element to generate the output power at another power P1′ (as the dotted line A). At the same time, in order to control the output power at the power P1, close loop feedback control is adopted to calculate the current for compensating the power difference (P1′−P1). Then, the calculation is referenced to increase the operating current (as the dotted line B) until the power generating element generates the output power at the power P1. At this time, the operating current is at a current I1′.
The prior art assumes that the relationship between the output power and operating current of the power generating element is represented by a line L2 when the temperature of the power generating element is at the temperature T2. If the power generating element is controlled to generate the output power at a power P2, the control module calculates the required current I2 for driving the power generating element to generate the output power at the power P2 according to the current I1 and a scale relationship (formula (1)) between the power P1′ and the power P2.
                              I          2          ′                =                                                            P                2                                            P                1                                      ⁢                          (                                                I                  1                  ′                                -                                  I                  1                                            )                                +                      I            2                                              (        1        )            
In fact, when the temperature of the power generating element is at a temperature T2, the output power and the operating current of the power generating element are actually related in a line L3 rather than the line L2. Therefore, the calculated operating current, which is equal to a current I2, drives the power generating element to generate the output power at the power P2′ (as the dotted line C). Thus, the power generating element does not correctly generate the output power at the power P2 and the output power is not precisely controlled at the temperature T2.
The power error caused by the current I.2 results from the prior technique's disregard of the threshold currents (intersections of the line L1, line L3 and the current axis). If the threshold current is ineffectual, the error caused by the prior technique is not obvious. However, if the threshold current is effectual or the output power is to be controlled more precisely, the operating current calculated according to the prior technique may include error.
Taking an optical disk drive, for example, an optical pickup head of the optical disk drive generates an output power for accessing the optical disk. Referring to FIG. 2 and FIG. 3, in the power control structure of the optical pickup head in the optical disk drive, an optical disk drive 1 includes an optical pickup head 10 and a power controller 15. The optical pickup head 10 includes a light-emitting module 11, a temperature measuring module 12, a light-sensing module 13, and a digital-to-analog converting module 14. The power controller 15 includes a power compensating module 16 and a power control module 17.
The temperature measuring module 12 measures the temperature of the light-emitting module 11 to generate a temperature signal 121. According to the temperature signal 121, the power controller 15 can control the light-emitting module 11 to prevent it from overheating. The light-sensing module 13 senses the output power of the light-emitting module 11 to generate a power feedback signal 131. The power compensating module 16 receives the power feedback signal 131 and calculates the power compensating current by a close loop feedback control. The power control module 17 generates a current control signal 171 according to the power compensating current in the above method. The digital-to-analog converting module 14 receives the current control signal 171 to generate an operating current 141. The operating current 141 drives the light-emitting module 11 to generate output powers P1 and P2. The output powers P1 and P2 are respectively a writing power and a reading power for accessing an optical disk.
However, if the temperature of the light-emitting module 11 rises, only the reading power is precisely controlled but the writing power is not due to the disregarded threshold currents. Therefore, the optical pickup head 10 does not correctly access the optical disk, and, even more, the lifetime of the optical pickup head 10 and the endurance of the optical disk drive 1 are reduced.
It is therefore a subject of the invention to provide an operating current modifying device and an operating current modifying method, which calculate the required current to drive the power generating element to generate the output power at different powers as the temperature of the power generating element varies.